1. Field of the Invention
The present invention relates to telecommunications, and, in particular, to subscriber-line interface circuits (SLICs) for telephones and other customer premises equipment (CPE).
2. Description of the Related Art
When located at a central office (CO), subscriber-line interface circuits provide the electrical interface between the CO and its customers' telephones and/or other CPE. FIG. 1 shows a high-level schematic diagram of a portion of a prior-art SLIC 100 used to support telecommunications services for an individual customer.
In particular, FIG. 1 shows tip amplifier 102 and ring amplifier 104, which are connected between the rest of the SLIC's audio-feeding circuitry 106 and tip and ring lines 108 and 110, respectively, which lines provide the electrical connection to the customer's CPE. Connected between tip line 108 and tip amp 102 is switch S3, while switch S4 switchably connects tip line 108 to ground. Similarly, connected between ring line 110 and ring amp 104 is switch S2, while switch S1 switchably connects ring line 110 to ground through ringing power supply 112, which, according to a conventional analog POTS (plain old telephone service) signaling protocol, provides ringing and a 48V DC voltage offset between ground and one side of switch S1. Depending on the implementation, the voltage applied to switch S1 by power supply 112 may be either positive or negative.
When the customer's CPE is on hook, the CPE is configured such that tip and ring lines 108 and 110 form an open circuit path through the CPE. When the customer's CPE is off hook, the configuration of CPE changes such that tip and ring lines 108 and 110 form a closed circuit path though the CPE. When a call is placed to the customer's CPE (e.g., when the CPE is on hook), SLIC 100 provides a ringing signal to the CPE via ring line 110, which signal returns to SLIC 100 (e.g., when the CPE goes off hook) via tip line 108.
During ringing, switches S1 and S4 are closed, and switches S2 and S3 are open. As such, the ringing voltage from power supply 112 is applied to the customer's CPE via ring line 110. The ringing signal is an intermittent AC signal (e.g., 88V rms) superimposed by power supply 112 on the 48V DC voltage offset.
When the CPE goes off hook (e.g., when the customer answer his/her phone), tip and ring lines 108 and 110 are connected via the CPE, which starts to draw DC current from power supply 112. Depending on the implementation, the ringing signal should be returned to the SLIC via tip line 108 either to ground or to battery. For example, in prior-art SLIC 100, the ringing signal is returned to ground via switch S4.
When the SLIC detects that the CPE is off hook (e.g., by detecting the DC current drawn from power supply 112), switches S1 and S4 are opened, and switches S2 and S3 are closed to connect tip and ring lines 108 and 110 to receive the audio signals provided by audio-feeding circuitry 106 via tip and ring amps 102 and 104, respectively.
In the past, switches S1-S4 were implemented using a double-pull, double-throw electro-mechanical relay having four poles. In more modern, integrated-circuit implementations, each of switches S1-S4 is implemented as a set of one or more relatively large transistors that are capable of handling the voltages involved in analog POTS service.